Always get the "master" kit. It contains everything needed to completely rebuild the head. The regular kit just includes the top stuff. For me, it is always the seals for the pedal and the motor that wear and leak. The other internals seem to wear well--no problems in 40 years (of course includes many rebuilds!). As these seals are the very last parts you reach you might as well replace all while you are there. Unless you really enjoy working on them! Also, the old rubber parts seem to take a set which makes it hard to stop small leaks if you reuse them.
There is a one-way duckbill valve at the outlet (the square part with four screws) which could cause the water to return if it is stuck open. Check this first before taking all apart.
I lube all parts with silicone grease which I get at a dive shop. They use it for underwater camera o-rings. Seems to last longer than engine room type grease and probably doesn't dissolve from toilet cleaners.
I replaced the studs holding the head to the deck with washers and wing nuts. This lets me unfasten the inlet and outlet hoses and take the entire head to a more convenient place to work on it than the floor.
These heads are pretty simple once you take one apart. Good luck!
